Titanium dioxide is being widely utilized for pigments, cosmetics such as UV creams, photocatalysts, solar cells and so on. It is also being broadly studied. Particularly, nano particles of titanium dioxide are attracting attention because of having various potential applications. Novel techniques relating to titanium dioxide include inserting europium ions (Eu3+) among nano sheets of titanium oxide to prepare a light emitting material that emits red light attributable to Eu3+ (see, inter alia, Non-Patent Document 1).
Another technique relating to titanium dioxide is a method of manufacturing ceramic powder directly from a mixture or the reaction product of a metal compound and an alkoxide by way of a thermal plasma process (see, inter alia, Patent Document 1).
FIG. 17 schematically illustrates the light emitting mechanism of a light emitting material prepared by inserting europium ions among nano sheets of titanium oxide.
According to known technique of the above cited Non-Patent Document 1, nano sheets of titanium oxide is irradiated with light to excite electrons in a valence band (VB) to move to a conduction band (CD). Electrons that are excited to move to the conduction band are not relaxed to move back directly to the valence band but trapped temporarily in a defect level. Later, such electrons move to the excitation level of Eu3+ without being recombined with holes in the valence band. As a result, it becomes possible to take out fluorescence attributable to Eu3+.
Such a move of energy from nano sheets of titanium oxide to Eu3+ takes place effectively when the energy level of the defect level of the nano sheets of titanium oxide is slightly higher than that of Eu3+ in an excited state. Since it is only necessary to excite nano sheets of titanium oxide that is a host compound in order to take out fluorescence attributable to rare earth ions, it is possible to use light of a wavelength that is absorbed by the nano sheets of titanium oxide.
On the other hand, Patent Document 1 describes a method of manufacturing ceramic powder, using (I) a metal compound other than alkoxide having one or more than one metal elements, (II) an alkoxide having one or more than one metal elements, a mixture of (I) and (I) and/or the reaction product of (I) and (II), by way of a thermal plasma process. With such a thermal plasma process, it is possible to obtain ceramic powder without using sintering and crushing steps.
With the technique described in Patent Document 1, it is possible to manufacture highly crystalline ceramic powder by using a precursor obtained by mixing (I) and (II) or causing them to react with each other as starting material.